July 8, 2013, 3:19 p.m.
EDMONTON - University of Alberta entomologist John Acorn was out counting butterflies this week with a student in MacKinnon Ravine, just southwest of the Royal Alberta Museum, when the pair walked toward a shaded bush and prompted a startling display.
“Maybe 100 dragonflies rushed out of this one rose bush where they’d been resting,” said Acorn, author of several books on bugs and host of the former TV series, Acorn, the Nature Nut. “This is where I do my regular butterfly survey and I’ve never ever seen Darner dragonfly numbers that high. They seem to be very common all around the city.”
Edmonton seems to be swarming with one of Alberta’s biggest and most common types of dragonflies, called the variable Darner, named for the blue, green and sometimes yellow patterns on its brown body. Acorn doesn’t count dragonflies, and neither does the city, but numbers this season seem to be soaring, he said.
“I first saw Darner dragonflies on June 17 and everything seemed normal until about Wednesday. You just couldn’t help but comment on it because they were everywhere, absolutely everywhere,” Acorn said.
Many Edmontonians posted messages this week on Twitter to say they’ve spotted large numbers of dragonflies and hope the carnivorous predators will gobble up plenty of mosquitoes.
Dragonflies eat all kind of flying insects, including flies, small butterflies and moths, Acorn said.
“There’s no doubt there would be more mosquitoes if they weren’t here,” he said. “Will they clean up all the mosquitoes? No.”
Heavy rains this year and a fairly wet summer last year likely gave the dragonfly population a boost. Adult dragonflies lay eggs in ponds and marshy lakes toward the end of each summer. The eggs hatch and then the larvae grow and spend winter under water, Acorn said. Dragonfly larvae need water to breathe and catch food by flicking out their segmented lower lip like a chameleon’s tongue to snag prey.
“The larvae are predators too. The larvae eat aquatic organisms in the ponds and the lakes, including mosquito larvae. They are pretty big. The larvae are up to about five centimetres long and they can even eat really small fish,” Acorn said. “How they get around is the coolest thing about them. They propel themselves forward by shooting water out of their rectum. They also have their gills in the rectal chamber and that’s how they breathe, is in and out the butt.”
Dragonfly larvae, called nymphs, eventually crawl out of the water and head skyward. Adult dragonflies have huge, compound eyes that contain thousands of lenses and can process a staggering amount of visual information. Keen vision coupled with front and rear wings that can move independently make dragonflies sophisticated flyers and skilled hunters, Acorn said.
“They live a long time, so these darners will be around well into September. They’re on the job for quite a few months.”
Another common dragonfly, the much smaller red or yellow cherry-faced meadowhawks, should also emerge soon, Acorn said. Those dragonflies hunt by perching on the ground or on low plants, darting out to catch their prey.
“If we get a big year for meadowhawks, which I imagine we will, we’ll be well served by them as nature’s pest control,” Acorn said. “Every once in a while you’ll see a darner eat a meadowhawk. The Darners are probably twice as big.”
City of Edmonton entomologist Peter Daly said dragonflies help control mosquitoes but won’t eradicate the biting pests. Mosquitoes make up a small part of the diet for dragonflies, which are “the terrors of the air” if you’re a smaller flying bug, Daly said.
“They are awesome predators. Some of them can fly reputedly at speeds close to 60 miles per hour in short bursts,” Daly said.
“We had a dragonfly nymph in a tray of water a few years ago and put 100 mosquito larvae in it and it had eaten all of them — all 100 — in under two minutes.”